A 35-year-old IV drug user presents with fever, a new regurgitant murmur at the tricuspid area, and multiple septic pulmonary emboli on CT. Blood cultures grow Staphylococcus aureus (MSSA). Echocardiography confirms 1.8 cm tricuspid valve vegetation. Which antibiotic regimen is most appropriate?

• A Oral rifampicin and ciprofloxacin for 4 weeks
• B IV vancomycin regardless of MSSA sensitivity
• C IV daptomycin plus gentamicin for 2 weeks only
• D IV oxacillin (or flucloxacillin) for 4–6 weeks ✓

Explanation

Right-sided (tricuspid) native valve endocarditis due to MSSA is treated with IV anti-staphylococcal beta-lactam (oxacillin or flucloxacillin) for 4–6 weeks, which is superior to vancomycin for MSSA (better bactericidal activity, lower failure rates). Vancomycin should be reserved for MRSA or true beta-lactam allergy. Oral rifampicin/ciprofloxacin is not standard for S. aureus NVE. Short 2-week courses with daptomycin + gentamicin have been studied in uncomplicated right-sided MSSA endocarditis in IV drug users without pulmonary complications.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.